The efficiency of electrophotographic type copiers or printers depends upon the proper relative charge being maintained between a photoconductive imaging member and a developing means. This charge relationship is relied upon to attract development material, i.e. toner, from a supply source to the photoconductive member in conformance with both the outline and density of the electrostatic image on the photoconductive member. The electrostatic image which undergoes this development may be formed through the expediency of exposing the previously charged photoconductive member to a light image of the original being copied or to exposure by an electro-optical exposure source.
One method of sustaining the proper charge relationship between the photoconductive member and the developing means is to use an electrostatic voltmeter, commonly called an electrometer, to sense potentials on the photoconductive member at some appropriate point. This device can be used as a service instrument to provide, by meter, a visible indication of the photoreceptor charge condition from which the electrostatic development field can be manually adjusted. In other cases, a feedback loop may be provided as part of a process control to enable readings from the electrometer to be used to automatically control the development field. Control over the electrostatic development field may be done, for example, by controlling or regulating the developer bias, or by controlling potentials on the photoconductor itself by regulating the corona charging means.
In machines of the type alluded to, adjustment of one or more of the various operating parameters, such as the primary charge potential level, normally requires that the bias be identified, and changes made therein monitored. Failing to monitor the bias, and changes thereto, may result in biases exceeding safe or designed maximum levels or problems in control of image quality.
Further, electrometers, like most test instruments, require calibration checks from time to time to assure that the readings obtained are accurate.
In U.S. Pat. No. 3,998,538, an electrometer probe is described wherein a probe support supports the probe. A further supporting structure is provided so that the probe can be mounted on the probe support at a fixed spatial relationship relative to a photoconductive imaging member. In order to check the probe for calibration it is necessary for a service person to remove the probe with its support from the supporting structure and position the probe and the probe support in a calibration receptacle. The receptacle includes a test plate electrode positioned such that the probe when seated in the receptacle is positioned from the test plate electrode in a preset spaced relationship. With a known electrical bias provided to the test plate electrode a reading of the electrometer can be used to calibrate same.
A problem with this approach is the need to remove the electrometer probe support from the supporting structure to another device which provides inconvenience and adds to the cost of service by requiring additional parts.
The aforementioned U.S. patent in an alternate embodiment describes a conductive test plate that is built into the reproduction machine at a location adjacent the imaging member. An electrometer probe is supported for translational movement to overlie the test plate for calibration purposes or to overlie the imaging member for sensing voltages or potentials on the imaging member. A problem with this device is that additional supporting structure is needed to accurately locate the test plate.
It is therefore an object of the present invention to provide a new and improved apparatus and method for calibrating and using electrometers with electrostatographic reproduction machines.
The invention and its various advantages will become more apparent to those skilled in the art from the ensuing detailed description of preferred embodiments, reference being made to the accompanying drawings.